Le vulcaniti permiane a N della Linea del Calisio: Evoluzione strutturale del margine SW del distretto vulcanico atesino (Trente, Italia)

A Lower Permian volcanic succession, cropping out between Calisio Line, Avisio Valley and Silla Creek (Trento area), has been studied from geological and petrographical point of view. The volcanites consist of andesitic lava flows followed by thin rhyodacitic ignimbrites, rhyolitic domes and rhyolitic ignimbrites. The petrographical and geochemical analysis show that the volcanites represent a medium- to high-K calc-alkaline suite. The isotopic evidence support a hybrid nature, resulting from a complex interaction between mantle-derived magmas and deep crustal materials. The succession is particularly reduced in thickness with respect to the ones of the adjoining sectors of the same volcanic district (Adige volcanic district, or AVD). In fact, the complessive thickness of the studied succession is of only 500 m, whereas a volcanic sequence of the same age, but of more than 1400 m in thickness, characterizes the close Pine Plateau. Comparing the different sequences we restore the palaeomorphology and Permian structural evolution, marked by strong synvolcanic extensional tectonics. The studied section represents a Permian structural high, that is named as Bosco High in this paper. The adjoining thicker succession of the Pine Plateau represents a Permian depression, named here as Pinè Depression. The difference between Bosco High and Pine Depression was settled by a Permian fault, that runs, at the present-day, along the Valley of Silla Creek. In our reconstruction, this was a normal fault dipping to SE, named here as Pinè Paleoline. A similar meaning has been assumed by the Calisio Line, which was the SW boundary of the AVD, during the Permian (Calisio Paleoline). During Neoalpine compressional tectonics, especially in the Serravallian-Tortonian Valsugana event, the Permian faults (Calisio and Pine Paleolines) were reactivated. The Calisio fault turn into a dextral strike-slip fault corresponding to the lateral ramp of the Valsugana Thrust. The Pine fault from Fornace to Le Piazze, where the Line is NE-SVV striking, acted as a NW-vergent backthrust, related to SSE-vergent Valsugana Thrust. From Fornace to Torchio, close its SW ending, the Pine Line is striking N-S and acted as sinistral transfer fault. This reactivation led to the complete inversion of the Permian normal throw. In the study area two sets of faults developed: NNE-SSW striking sinistral strike-slip faults and NVV-SE striking dextral ones, indicating a NNW-SSE trending compressional axis related to the Valsugana event.